1. Field of the Invention
The invention relates generally to an improved substrate support apparatus and more specifically to an electrostatic chuck with barrier enhanced electrodes for reducing the charge accumulation phenomenon within the chuck.
2. Description of the Background Art
Electrostatic chucks are used for retaining a workpiece in a variety of applications including securing a substrate (i.e., a semiconductor wafer) within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for securing or clamping wafers to a pedestal located within the process chamber. The pedestal is provided with additional equipment, e.g., heaters, heat sinks, heat transfer gas ports, additional electrodes and the like to regulate temperature, electrical bias and other chamber conditions during wafer processing to optimize yield. During a typical wafer processing cycle, a wafer is introduced to a process chamber and disposed on a support surface of the electrostatic chuck. The electrodes are energized with a chucking voltage for a period of time which can be from a few milliseconds to up to 30 seconds during which wafer processing (i.e., physical vapor deposition (PVD), chemical vapor deposition (CVD), chemical mechanical polishing (CMP) or the like) is performed. Subsequently, the electrodes are de-energized and the wafer removed from the process chamber.
Ideally, there are no residual or accumulated charges retained in the electrostatic chuck when the electrodes are de-energized. As such, the wafer is easily removed from the support surface of the electrostatic chuck. In other words, a residual charge accumulation in the electrostatic chuck creates a residual chucking force that must be overcome before the wafer can be removed from the chamber. If excessive force is used to overcome the residual chucking force, the wafer may crack, break, have damaged circuit devices thereupon or the like. Residual charges are also detrimental because they reduce the available chucking force for retaining a wafer on the support surface. This condition, in turn, results in poor process conditions. For example, a reduced chucking force can contribute to a non-uniform heat transfer gas pressure applied to the backside of the wafer. Such unequal forces cause wafer shifting or pop-off and compromises temperature control which results in poor process conditions or particle contamination. Additionally, during the course of batch processing, it becomes increasingly difficult to "dechuck" or remove a processed wafer due to the accumulation of these charges in the electrostatic chuck.
FIG. 1 depicts a typical, bipolar electrostatic chuck 100 used for supporting and retaining a substrate such as a semiconductor wafer 106 in a process chamber (not shown). The chuck 100 has a chuck body 102 usually formed of a dielectric material such as aluminum nitride or boron nitride with one or more electrodes 108.sub.1 and 108.sub.2 disposed within the chuck body 102. A power source 110 is connected to and oppositely biases each of the electrodes to establish the desired electrostatic field and chucking force. As such, the wafer 106 is retained upon a support surface 104 of the electrostatic chuck.
In some instances, the electrodes are fabricated from molybdenum which is a desired material because of its electrical properties and relatively close thermal expansion coefficient with respect to the aluminum nitride chuck body. However, it has been noticed that when a negative potential is applied to a molybdenum-based electrode in an aluminum nitride body, the potential at the support surface above the negative electrode degrades or is otherwise reduced with respect to the potential applied to the positive electrode and seen at the support surface above the positive electrode.
FIG. 2 depicts this phenomenon in greater detail. Specifically, FIG. 2 depicts a graph 200 of time (in seconds) vs. potential (in volts) at the support surface 104 above the positive and negative electrodes of electrostatic chuck 100. Section A of the graph 200 corresponds to the period of time t where 0&lt;t&lt;T1 and the chucking voltage applied to the electrodes is zero. Section B corresponds to a time t where T1&lt;t&lt;T2 where a positive chucking voltage is applied to the positive electrode 108.sub.1 (line 202) and a negative chucking voltage is applied to the negative electrode 108.sub.2 (line 204). As seen, once the chucking voltage is applied, the positive voltage maintains a steady state value. The negative voltage however, first reaches a value approximately equal in magnitude to the positive value (but opposite in sign of course) and then decays to a less negative (i.e., more positive) value. This change in potential at the support surface is denoted as .increment.V.sub.n. Section C corresponds to a time t where t&gt;T2 and the chucking voltage has been turned off. The positive voltage rapidly drops to zero and may have a slight, but relatively insignificant residual value. The negative value however, overshoots the Y=O axis by a value of approximately .increment.V.sub.n and begins a slow decay.
The reduced negative potential seen in Section B is undesirable because of reduced chucking force at the surface. This same condition is undesirable in Section C because of the inability to rapidly remove (or "dechuck") a wafer from the support surface. Additionally, it has been noted that disconnecting power to or temporarily reversing the polarity of electrodes does not relieve this condition. It is suggested that the value .increment.V.sub.n is created by an electrochemical process between the electrode/chuck body interface so that a localized positive charge continuously exists within the chuck body. In effect, the proximity of the molybdenum electrodes to the aluminum nitride chuck body creates an accumulated charge or internal battery condition that is deleterious to electrostatic chucking forces. Specifically, a repelling force created by the positive charge at what should be a negatively charged region compromises chucking forces and may even force the wafer off the substrate support entirely.
Therefore, there is a need in the art for an improved electrostatic chuck that is not subject to undesirable charge accumulation after repeated use.